WASHINGTON — President Obama, acknowledging the country’s weariness with war, said Saturday that the United States should use military force to punish Syria for using chemical weapons but said he will first seek congressional approval.

The surprise announcement, following a series of consultations with senior congressional leaders, effectively delayed any US military operation in Syria until after the summer recess, which ends on Sept. 9.

It marked a reversal from what appeared to be Obama’s willingness to assert his presidential authority without a green light from the legislative branch.

Obama’s decision upped the political stakes for a leader who has had little success this year winning over a recalcitrant Congress, particularly the Republican-controlled House of Representatives. Coming on the heels of the defeat of a similar measure in the British House of Commons last week, it was also seen as a gamble that raises the possibility the president may have to cancel his plans for a strike if Congress balks.

“Having made my decision as commander-in-chief based on what I am convinced is our national security interests, I’m also mindful that I’m the president of the world’s oldest constitutional democracy,” Obama said. “I’ve long believed that our power is rooted not just in our military might, but in our example as a government of the people, by the people, and for the people. And that’s why I’ve made a second decision: I will seek authorization for the use of force from the American people’s representatives in Congress.”

“The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation,” Obama said in 2007, adding, “In instances of self-defense, the president would be within his constitutional authority to act before advising Congress or seeking its consent.

“History has shown us time and again, however, that military action is most successful when it is authorized and supported by the Legislative branch,’’ he said then. “It is always preferable to have the informed consent of Congress prior to any military action.”

On Saturday, congressional leaders praised Obama for seeking approval from the legislative branch but it was far from certain he would get the backing he is seeking.

House Speaker John Boehner — who had pushed Obama to answer more questions on Syria — joined top House Republican leaders in praising the announcement on Saturday, while not indicating whether they would support a resolution approving the use of force.

“Under the Constitution, the responsibility to declare war lies with Congress,” the Ohio Republican said in a statement. “We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised.”

Many members of Congress issued equivocal statements, praising the decision to hold a vote but leaving it unclear whether they would approve a strike. Some lawmakers made plans to return to Washington for high-level briefings that will take place in person, but the votes aren’t likely to happen for at least a week.

Some of the president’s closest political allies may need the most persuading. Key members of the Massachusetts delegation sounded wary about US military strikes on Syria.

“While Syria needs to be held accountable for its outrageous actions, I still believe that every diplomatic option ought to be exhausted before the United States and its international partners contemplate any military action in the region,” said Representative Richard E. Neal, a Springfield Democrat and dean of the Massachusetts delegation.

Bay State lawmakers were among the most vociferous opponents of the US-led invasion of Iraq in 2003. Neal voted against the Iraq War.

Speaking in the Rose Garden, with Vice President Joe Biden at his side, Obama on Saturday voiced his strongest condemnation yet of the regime of Syrian President Bashar Assad for allegedly using poison gas to kill more than 1,400 civilians, asserting that “in a world with many dangers this menace must be confronted.”

If the United States fails to act, he added, “it could lead to escalating use of chemical weapons, or their proliferation to terrorist groups who would do our people harm.” Obama said he has been assured by military leaders that a strike would be effective even if it is not launched for a month.

On Saturday evening the White House sent to Congress draft legislation seeking authorization to use the armed forces to “prevent or deter the use or proliferation” of “any weapons of mass destruction’’ including chemical weapons by Syria. It would also authorize use of force to “protect the United States and its allies and partners against the threat posed by such weapons.’’

Some analysts criticized the president for his apparent change of heart about seeking a congressional vote, even as it is clear he is prepared to act regardless.

“It is sort of clunky national security strategy,” said retired Major General Robert Scales, a former commandant of the Army War College. “The Syrian Army now has an additional nine or 10 days to disperse their strategic assets, perhaps to transport them to Iran.”

Scales, who said he believes any military intervention in Syria “is not in America’s strategic interest,” worried that the Syrian leadership would interpret Obama’s decision to seek a congressional vote “as something of a victory.”

Obama will now take his case to a divided Congress that has antagonized him on most every other issue, even routine matters such as federal budgets, judicial nominees, and a farm bill.

That process began Saturday during conference calls conducted for all senators by top administration officials, including Secretary of State John Kerry and National Security Adviser Susan Rice. Kerry is slated to make the administration’s case during a round of appearances on Sunday morning talk shows.

The decision to seek congressional support drew praise from members of both parties.

“That is good for every American, especially, I believe, for those who wear our nation’s uniform and would be called upon to execute any order that the president might issue,” said Representative Scott Rigell, a Virginia Republican who last week enlisted more than 100 members to sign a bipartisan letter urging Obama to seek a vote in Congress before acting in Syria.

Senator Johnny Isakson, a Republican from Georgia, said he supported Obama’s decision to use force but said the president should have called Congress back into session immediately.

“If we fail to take strong action against Syria for this horrendous attack, then we are sending a signal to Syria as well as to Iran and North Korea that they are accountable to no one,” he said.

Senator Carl Levin, the Michigan Democrat who chairs the Armed Services Committee, said Obama “wisely chose to seek congressional support, even though he believes he is not required by law to do so.”

Just as Obama will take his case to Congress and the public, opponents of his proposal plan to do the same.

Protesters demonstrated loudly outside the White House gates as Obama made his remarks in the Rose Garden. In Boston, several hundred people marched from the Park Street MBTA station to Faneuil Hall.

“Neither bombing Syria, nor any other overt act of war, will help the people of Syria,” said Garrett Kirkland, one of the Boston rally’s organizers.

Members of Congress from Massachusetts, like those around the country, said they plan to study the intelligence and listen to constituents.

Representative Michael Capuano, who opposed the Iraq War, said he is more inclined to support military action in this case because chemical weapons were used. But he wants to hear more about what the potential consequences might be.

“The question is, what happens the day after?” asked the Somerville Democrat. “And that question has not been answered yet. Are we empowering Al Qaeda? Are we empowering Hezbollah?”

Representative Stephen Lynch, a South Boston Democrat, said he is concerned about the lack of an international coalition and that US military action could potentially strengthen groups linked to Al Qaeda involved in the two-year civil war to topple the Assad regime.

Representative James McGovern, a Worcester Democrat, also expressed reservations about the president’s stance, saying, “
I just don’t know, in the long-term, if trying to go after certain military targets makes it more likely or less likely that this terrible violence can come to an end.”

Senator Edward J. Markey, a Massachusetts Democrat, who earlier in the week said he would support a surgical strike to limit Assad’s ability to use his large supply of poison gas, sounded noncommittal on Saturday.

“The aftermath of a US strike on targets in Syria is difficult to predict, with negative consequences that may be beyond our capability to control,” he said in a statement.

Senator Elizabeth Warren, a Massachusetts Democrat, also was skeptical, saying “it is critical that we recognize the complexity of the conflict on the ground and that we consider the potential for unintended consequences of US intervention, no matter how good our intentions.”

Obama sought to address a major question he is certain to be asked as he tries to win over members of Congress, saying, “This would not be an open-ended intervention. We would not put boots on the ground. Instead, our action would be designed to be limited in duration and scope.”

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